A tandem type electro-photographic color printer is known in which photosensitive drums for the colors of cyan, yellow, magenta and black are juxtaposed with one another. In such a conventional tandem type color printer, four photosensitive drums integrally mounted to or detached from a main body casing are proposed.
More specifically, a drum unit or holder unit is provided including a plurality of drum sub-units and a pair of side plates. Each drum sub-unit is configured to support each photosensitive drum, and each side plate is positioned at each axial end portion of the drum sub unit to nip the drum sub-units between the pair of side plates. A plurality of developer cartridges or developing unit each holding a developing roller and each in association with each drum sub-unit are detachably attached to the drum unit. Such drum unit is detachably mounted on the main body casing as described in Japanese Patent Application Publication No. 2007-72422.